Stencil printing machines are known which provide an adjustment arrangement for material gripping and displacement arrangements, usually designated as gripper beams. This adjustment arrangement is intended to impart a precise position to the gripper beams in relation to a printing table in the printing machine. The gripper beams include members for gripping and displacing a material. For their movement the gripper beams interact with at least one, preferably two, endless feed chains or the like, which are arranged in parallel. These cease to move when the gripper beams are located in predetermined positions, one for the gripper beam to grip a sheet or material intended for printing, and one for the gripper beam to hold or leave a sheet in a position for application of print to the sheet. The gripper beams can be made adjustable by means of a double lever arm system so that the gripper beams adopt precisely the said predetermined positions, by providing the end surfaces of the gripper beams and the outer ends of a double lever arm system with interacting members.
Here the members comprise on the one hand convex, preferably cylindrically or spherical surfaces and secondly preferably "V"-shaped recesses which are intended to be pressed against each other in the predetermined position.
An adjustment arrangement of the type mentioned above has been described previously in British Patent Specification No. 1 208 614.
Further it is previously known from U.S. Pat. No. 4,031,824 to use a single elongated feed gripper member and a single elongated delivery gripper member, which are mounted in spaced relationship on a transfer carriage assembly movable relative to the printing bed of a printing press on a frame having a pair of parallel spaced rails. The feed gripper member and delivery gripper member are maintained in spaced, parallel relationship near opposite ends of the transfer carriage and move with the transfer carriage as a single unit relative to the printing bed.
In operation, the stock to be printed may be first registered to the outside of the printing bed of the press and the feed gripper member, being mounted on a spring-biased cam-operated mounting plate moved horizontally and vertically relative to the stock to be printed to pick it up by vacuum in a registered position. The transfer carriage then moves along the rails of the frame so that the stock is brought into a printing position by the feed gripper. As the feed gripper is moving toward the printing bed, the delivery gripper, consisting of a pair of elongated, parallel jaws which open in opposite directions about a common pivot pin, has simultaneously engaged and gripped a protruding edge of a sheet of printed stock and begun to transfer such stock to a delivery position.
After feeding and delivery has been accomplished, the transfer carriage may be quickly returned to its initial position and begin to repeat the process. The operation of the printing press is preferably timed so that printing is accomplished during return of the transfer carriage.
It is already known that the drive for the gripper beams described above with endless drive chains arranged in parallel becomes extremely complicated, because it is not only a question of driving the chains intermittently so that the gripper beams stop in predetermined positions, but furthermore tensioning arrangements are required for the chains and also other mechanical members. Furthermore it is known that a drive arrangement for gripper beams of the type mentioned above has to be dimensioned for high power input, because the construction as a whole becomes heavy and the acceleration and retardation moments require high power inputs.
There has long been a desire to be able to create conditions such that the drive arrangement for the gripper beams is made simpler, while at the same time the gripper beam design is made light and simple, thereby making it possible to displace the gripper beams rapidly between two predetermined positions without excessive consumption of power.
Furthermore it is a difficult technical problem to create conditions such that the transport speed of the gripper beams is easily capable of regulation, for example, so that the gripper beam is subjected to high acceleration, high velocity and a somewhat reduced retardation.
Furthermore there has been a desire, and this represents a difficult technical problem, to create conditions such that, in connection with stencil printing machines, the material provided with a printed image can be delivered easily. It is particularly advantageous if special delivery belts and special delivery fingers can be eliminated.
A particular problem which has been encountered with stencil printing machines, especially with such stencil printing machines that function at high printing speeds, is that the insertion time for material intended to be printed to reach the insertion position is short and this becomes particularly difficult when insertion is carried out manually.